hi all, just wondering if I could get a second or third opinion on a quick repair I'm doing on a JL Audio 450/4 v1.
Usual story, bought the amp, seller said it worked fine, immediate low ohm light on with the power light, had low output with high distortion on channel 2 and some distortion on channel 1. 3 and 4 are fine. Bias pots on any channel do not appear to have been adjusted.
Opened it up, found a shorted transistor @ Q209. Out of circuit, dead shorts on all three legs on the diode test. Missing R246 resistor, this goes back directly to Q209, assuming it popped and someone removed it? R249, 47 ohm gate resistor is open circuit. Q202 and Q203 were getting hot (approx 100-110F after 5 minutes at idle) with no load or input, idle current on the amp was 8-9 amps. Q202, Q203 were my initial guesses at the issue because of the heat there, so they were removed which turned off the low ohm light and made channel 1 distortion go away.
Still had extremely low output with a lot of distortion on channel 2, which led me to Q208 and Q209.
Also, as a side note, the positive terminal on this amp had desoldered itself and melted the plastic casing around it. It is reseated now, but I wanted to add it just in case it was relevant. I would guess it was high junction resistance/small wiring which created the heat.
I already resoldered the 8x 47k resistors on the audio driver boards - noticed some small solder cracks and wanted to preemptively take care of that.
Questions:
Aside from replacing Q202/203/208/209, the 47 ohm gate resistor, and R246 resistor (0.1 ohm 5w), are there any other sanity checks I should do to verify that the issues are limited to these components? I believe the 450/4 has 4 FET's per channel on 1 & 2, and 2 FET's per channel on 3 & 4 so I should be replacing all 4 for channel 2.
Thanks in advance for your help
Usual story, bought the amp, seller said it worked fine, immediate low ohm light on with the power light, had low output with high distortion on channel 2 and some distortion on channel 1. 3 and 4 are fine. Bias pots on any channel do not appear to have been adjusted.
Opened it up, found a shorted transistor @ Q209. Out of circuit, dead shorts on all three legs on the diode test. Missing R246 resistor, this goes back directly to Q209, assuming it popped and someone removed it? R249, 47 ohm gate resistor is open circuit. Q202 and Q203 were getting hot (approx 100-110F after 5 minutes at idle) with no load or input, idle current on the amp was 8-9 amps. Q202, Q203 were my initial guesses at the issue because of the heat there, so they were removed which turned off the low ohm light and made channel 1 distortion go away.
Still had extremely low output with a lot of distortion on channel 2, which led me to Q208 and Q209.
Also, as a side note, the positive terminal on this amp had desoldered itself and melted the plastic casing around it. It is reseated now, but I wanted to add it just in case it was relevant. I would guess it was high junction resistance/small wiring which created the heat.
I already resoldered the 8x 47k resistors on the audio driver boards - noticed some small solder cracks and wanted to preemptively take care of that.
Questions:
Aside from replacing Q202/203/208/209, the 47 ohm gate resistor, and R246 resistor (0.1 ohm 5w), are there any other sanity checks I should do to verify that the issues are limited to these components? I believe the 450/4 has 4 FET's per channel on 1 & 2, and 2 FET's per channel on 3 & 4 so I should be replacing all 4 for channel 2.
Thanks in advance for your help
I would replace all 4 outputs in that channel.
For the missing source resistor, I'd replace the one that's working in 'parallel' with it as well. If it's idling at 8 amps, the outputs are likely operating at high temperature without clamping them down. I'd set the bias pots fully CCW until you get the amp working.
For the missing source resistor, I'd replace the one that's working in 'parallel' with it as well. If it's idling at 8 amps, the outputs are likely operating at high temperature without clamping them down. I'd set the bias pots fully CCW until you get the amp working.
I have one of these amps in for repair right now. Should be an easy fix, it works just doesn't turn off when the remote is turned off. So if you need to verify anything, just ask and I will check it or get you a picture.
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